We ONLY fill tanks, we do NOT exchange them, So if you pay for 20 pounds of Propane, you get 20 pounds. Unlike Tank exchanges at gas stations and grocery stores, where you ONLY get 15 lbs. Check their signs, you're getting ripped off. Plus by exchanging, you could be getting an older outdated tank instead of your tank that you know is safe. Also, we only charge for what we put in, no flat fees, no minimum charge!